1. Field of the Invention
The present invention relates to a plasma CVD (chemical vapor deposition) apparatus of the capacitance coupling type. More particularly, it relates to an apparatus for making a photoreceptor member used in an electrophotographic copying machine.
2. Description of the Prior Art
The forming of a photosensitive layer of amorphous silicone on a plate or drum is generally done by the decomposition of monosilane (SiH.sub.4) gas or disilane (Si.sub.2 H.sub.6) gas through a plasma CVD method. In the plasma CVD method it is necessary to supply a high frequency (RF) power for the generation of plasma, and such a supply is done by a capacitance coupling or inductance coupling. From the viewpoint of mass production of the photoreceptor member, it has been considered that it is appropriate to use the capacitance coupling rather than inductance coupling.
The plasma CVD apparatus of capacitance coupling type according to the prior art is shown in FIGS. 1 and 2, which is designed for depositing a photosensitive layer on a surface of a drum 1 thereby making a photoreceptor drum for use in an electrophotographic copying machine. The apparatus shown includes a cylindrical chamber 3 made of metal and a cylindrical electrode 2 provided in chamber 3. Suitable pipes P1 and P2 are connected to the chamber so as to evacuate the chamber and also to supply a suitable amount of monosilane or disilane gas.
Electrode 2 is connected to an RF power supply 6 through a suitable matching unit 5 of capacitive type. A matching unit is disclosed, for example, in "Experimental and design information for calculating impedance matching networks for use in RF sputtering and plasma chemistry" presented in "Vacuum" volume 29, number 10.
When the RF power is supplied, the plasma extends not only between electrode 2 and drum 1, but also between electrode 2 and the inside wall of chamber 3.
An improvement is shown in FIG. 3 in which a cylindrical shield plate 4 is provided around electrode 2 with a spacing of about several millimeters between electrode 2 and plate 4. The electric potential of plate 4 is made the same as that of chamber 3 and, therefore, no plasma will be produced between electrode 2 and the inner wall of chamber 3.
In the plasma CVD apparatus, the chamber is evacuated to a certain degree, and a certain amount of reaction gas is introduced into the chamber. Therefore, the volume of the chamber is a very important factor for determining the amount of gas necessary for the reaction process. In any one of the above described apparatus, however, electrode 2 is provided independently inside chamber 3 and, therefore, the chamber volume is much greater than that is necessary to generate plasma. Thus, more than necessary reaction gas is supplied to the chamber. Also, the size of chamber 3 is bulky.
To eliminate the above described disadvantages, an attempt has been made to remove electrode 2 and shield 4, and instead to provide an electrode on the inside wall of chamber 3. In other words, the chamber itself is supplied with the RF power. However, this introduces a further problem to be solved, such that chamber 3 must be electrically isolated. To this end, it is necessary to electrically insulate chamber 3, and also to provide a suitable insulating means in each pipes P1 and P2 so as to accomplish the complete isolation of the chamber which is supplied with RF power. Furthermore, from a safety viewpoint, it is necessary to provide a suitable isolation means so as to prevent any workers from having access to the chamber while it is operating.